I personally bought it because it looked like some innovation on the 2D Mario formula, which -to me- had become stale with New Super Mario Bros. The little I played multiplayer, I didn’t find it particularly great, mostly when players have very different level of skill. I think this might be a case of “correlation does not imply causation.” But what do I know 🤷
What’s a safe bet is -as the article says- the movie definitely helped boost sales, as it did with older titles like Mario Kart 8 Deluxe.
I fully agree with your assessment. The game had a great trailer, a groovy vibe, and was right on the back of the Mario movie. It would have been almost exactly as successful with the classic turn-taking multiplayer experience.
It was cool to be able to play with friends and see the ghosts of randos, but it was a minor novelty, not a major selling point.